1. Field of the Invention
The present invention relates to an apparatus for increasing the strength of a surface of metal components.
2. Description of the Related Art
Some metal components, such as gears, are required to have their surfaces improved for fatigue strength because they are subject to repetitive loads in use. One known process of hardening the surface of gears is a shot peening process which blasts the gear surface with steel balls or the like to give compressive residual stresses to the gear surface.
The conventional shot peening process which employs hard steel balls is disadvantageous in that the hard steel balls tend to roughen the gear surface, lowering the smoothness thereof. Another known shot peening process, which is disclosed in Japanese patent publication No. 5-21711, applies glass beads having diameters ranging from 0.2 to 0.6 mm to the surface of metal parts. One problem of the disclosed shot peening process is that the glass beads are ejected in a poor directivity pattern, i.e., ejected in various different directions, and hence are not used efficiently.
There has been proposed a method of increasing the strength of the surface of metal components for reliably improving surface roughness and fatigue strength thereof with increased working efficiency, as disclosed in Japanese laid-open patent publication No. 8-73930. According to the proposed method, a liquid mixed with glass beads is applied to a metal component to increase the toughness of its surface. Because the glass beads are mixed with the liquid, the glass beads are impelled in a desired directivity pattern to strike a desired surface area of the metal component, which is hardened into an improved surface layer kept under residual compression.
According to the conventional shot peening processes, metal components are peened in a processing chamber. Immediately after a metal component is peened in the processing chamber, a mist containing crushed fragments of the glass beads is suspended in the processing chamber. In order to prevent the mist from flowing out of the processing chamber, it is necessary to remove the processed metal component from the processing chamber a predetermined period of time after the metal component has been peened. This practice has made a peening cycle time considerably long.
The peened metal component needs to be washed to remove crushed glass bead debris therefrom, and thereafter to be dried for rust prevention. These washing and drying steps that are individually carried out following the peening process are another factor that also makes the entire cycle time longer.
The glass beads are usually stored in a hopper and fed at a certain rate to an impelling nozzle through a tube that interconnects the hopper and the impelling nozzle.
Since the hopper is disposed in a relatively high position above the nozzle, the rate at which the glass beads are supplied from the hopper to the nozzle is liable to vary depending on the quantity of glass beads stored in the hopper. The varying rate prevents glass beads from being applied at a constant rate to metal parts, with the result that the metal parts cannot be blasted with the stable shot for desired surface strength.
The hopper installed in the high position also prevents itself from increasing its volume in an upward direction because of limitations imposed on the worker who handles the hopper for replenishing it with glass beads, servicing the hopper, etc. As a consequence, the volume of the hopper cannot be increased as desired.
The nozzle for impelling the liquid mixed with glass beads is positioned relatively closely to the metal component that is held in a blasting station because of the need to apply the mixed stream of the liquid and glass beads reliably to a desired surface area of the metal component.
When a metal component is automatically taken into and out of the blasting station by a feed mechanism, the feed mechanism may possibly be brought into physical interference with the impelling nozzle. To avoid such physical interference, it is necessary to additionally provide a mechanism for positioning the impelling nozzle with respect to the metal component and a mechanism for moving the impelling nozzle to a position out of physical interference with the feed mechanism. As a result, the overall facility for shot peening is complex and large in size.